Polycarbonates are a general-purpose engineering thermoplastic and have found a wide range of applications, particularly in injection molding or a glass sheet as an alternative to a windowpane.
Interfacial polycondensation, transesterification and other methods have hitherto been applied to the production of these polycarbonates. Although interfacial polycondensation is generally effective for the production of a polycarbonate, it has drawbacks such as the use of toxic phosgene as a starting compound for the reaction and the remaining of chlorine ions generated during the reaction in the resultant polycarbonate.
In order to eliminate the above-described drawbacks of the interfacial polycondensation, Japanese Patent Publication-A No. 182336/1988 discloses a process for producing a polycarbonate wherein a liquid trichloromethyl chloroformate as a dimer of phosgene is used instead of the toxic phosgene and subjected to interfacial polycondensation with a special dihydric phenol. This document, however, cities only 9,9-bis(4-hydroxyphenyl)fluorenes as the special dihydric phenol.
Angew. Chem., 99, 922 (1987) and German Patent DE 3440141 describe a process for producing a polycarbonate wherein triphosgene is used instead of the toxic phosgene and reacted with 2,2-bis(4-hydroxyphenyl)propane. This process, however, involves also a reaction mechanism whereby phosgene is generated.
In a process for producing a polycarbonate known in the art regarding the transesterification, a high-molecular-weight polycarbonate is produced by adding a transesterification catalyst to diphenyl carbonate and an aromatic dihydroxy compound as the starting compounds for the reaction, heating the mixture under reduced pressure to prepare a prepolymer while distilling off phenol and finally heating the prepolymer to 290.degree. C. or above in a high vacuum to distill off phenol (see U.S. Pat. No. 4,345,062). However, it is known that, unlike other engineering plastics, the melt viscosity of the high-molecular-weight polycarbonate is so high that the reaction should be conducted at a temperature as high as 290.degree. C. or above and that a high vacuum (10.sup.-2 Torr) is necessary for distilling off phenol having a high boiling point, so that the industrialization of this process is difficult also from the viewpoint of facilities, and the remaining of phenol in the resultant polycarbonate has an adverse effect on the hue and properties of the polycarbonate.
However, various studies have been made on the transesterification because it can be conducted through melt polycondensation and is a technique excellent in the profitability from the viewpoint of industry. In particular, since the viscosity of the reaction system becomes high when the polycondensation approaches completion, attempts have been made to use various types of apparatuses for the purpose of treating a reaction product having a high viscosity (Japanese Patent Publication-B No. 36159/1977 and Japanese Patent Publication-A Nos. 86618/1990 and 153923 to 153927/1990).
In the Japanese Patent Publication-B No. 36159/1977, the use of an intermeshing twin-screw extruder of a screw evaporator type is disclosed. This extruder, however, has problems of liability to form a black foreign matter in the residence section of screw grooves, the difficulty of controlling the residence time, the occurrence of coloring of the product due to the heating and the difficulty of efficiently removing distillates such as phenol as a by-product.
In the Japanese Patent publication-A Nos. 153923 to 153927/1990, use is made of a horizontal agitating polymerization tank. When this tank is used, although a large hold-up volume can be attained, the solution thickness becomes so large that it becomes difficult to distill off phenol as the by-product from the reaction mixture having a high viscosity. This determines the rate of the reaction, and consequently the residence time should be prolonged. A long residence time means that the formed polycarbonate is exposed to a high temperature for a long period of time, which is causative of the coloring of the reaction product.
Thus, the conventional transesterification process is unsatisfactory for efficiently producing a colorless, transparent polycarbonate having a high molecular weight on an industrial scale.